1. Field of the Invention
This invention relates to vehicle suspension systems, and more particularly to a method and apparatus for determining the relative velocity between the telescopically movable components of a hydraulic damping device.
2. Description of Related Art
Damping devices ("dampers") are used in conjunction with automotive suspension systems to absorb unwanted vibration which occurs during driving. To absorb this unwanted vibration, dampers are generally connected between the sprung mass ("body") and the unsprung mass ("wheel") of the automobile. A piston is located within the damper and is connected usually to the body of the automobile through a piston rod. Because the piston valving and orifices act to restrict the flow of damping fluid within the working chamber of the damper when the damper is compressed, the damper is able to produce a damping force which counteracts the motion of the wheel and/or body which would otherwise remain undamped. The greater the degree to which the flow of damping fluid within the working chamber is restricted by the piston, the greater the damping forces which are generated by the damper.
In selecting the amount of damping that a damper is to provide, three vehicle performance characteristics are often considered: ride comfort, vehicle handling and road holding ability. Ride comfort is often a function of the spring constant of the main suspension springs of the vehicle, as well as the spring constant of the seat, tires, and the dampers. Vehicle handling is related, among other things, to variation in the body's attitude (i.e., roll, pitch and yaw). For optimum vehicle handling, relatively large damping forces are required to avoid excessively rapid variation in the body's attitude during cornering, acceleration, and deceleration. Road holding ability is generally a function of the amount of variation in the normal load between the tires and the ground. To optimize road holding ability, larger damping forces are required when driving on irregular surfaces to minimize the normal load variations and to prevent complete loss of contact between the wheels and the ground.
To optimize ride comfort, vehicle handling, and road holding ability, it is generally desirable to have the damping forces generated by the damper be responsive to the frequency of the input from the road or from the body. When the input frequency is approximately equal to a natural frequency of the body (e.g., approximately between 1-2 Hz), it is generally desirable to have the damper provide relatively large damping forces (relative to critical damping) to avoid excessively rapid variation of the vehicle's attitude during cornering, acceleration and deceleration. When the input frequency is between 2-10 Hz mostly from the road, it is generally desirable to have the damper provide low damping levels so as to produce a smooth ride and allow the wheels to follow changes in road elevation. When the input frequency from the road is approximately equal to the natural frequency of the automobile suspension (i.e., approximately 10-15 Hz), it is desirable on one hand to have relatively low damping forces to provide a smooth ride, and on the other hand provide high damping forces so as to minimize variation in tire normal load and prevent complete loss of contact between the wheels and the ground.
Various methods are known for selectively changing the damping characteristics of a damper in response to an input frequency from the road. PCT application No. PCT/US 87/00615 discloses one such method. The apparatus used to perform the method comprises a pressure cylinder forming a working chamber having first and second portions operable to store damping fluid. The apparatus further comprises a first valve for controlling the flow of damping fluid between the first and second portions of the working chamber during compression of the apparatus. In addition, the apparatus also comprises a pressure chamber in fluid communication with the first portion of the working chamber and the first valve. A solenoid is also provided for regulating the flow of damping fluid between the pressure chamber and the second portion of the working chamber. A second valve is further provided for controlling the flow of damping fluid between the first and second portions of the working chamber during rebound of the apparatus.
When such methods are used for changing the damping characteristics of a damper, they often require information regarding the movement of the piston within the pressure cylinder of the damper. This information not only identifies whether the damper is in compression or extension, but also can provide information concerning the magnitude and frequency of suspension motion.
Several methods are known for obtaining information regarding the movement of the piston within the pressure cylinder. PCT application No. PCT/US87/00615 uses a pressure sensor as well as an accelerometer to determine whether the damper is in compression or extension, as well as to obtain information regarding the road surface. U.K. Pat. application No. GB 2 177 475A and West German Pat. No. G 87 02 817.4 disclose suspension damping devices incorporating ultrasonic wave systems for determining positional displacement information. The positional displacement information is obtained by determining the time from transmission of an ultrasonic wave to when its reflected "echo" wave is received. Both references use a single transducer acting to emit and receive the pulsed ultrasonic waves. Use of a single transducer necessitates incorporation of costly ultrsonic wave modulation and calibration circuitry to ensure coherent wave detection. Additionally, the transducers in both references are mounted such that the piston acts to reflect the ultrasonic waves.